Acute phase reaction is a fast, unspecific and highly complex reaction of the animal organism to injury and infection. Main symptoms and metabolic changes are fever, leucocytosis, dramatic rearrangement of plasma protein synthesis in the liver and of the protein synthesis in several other organs. The identification of the liver as main source of plasma proteins lead to the assumption, that the host reaction to the injury, and not the injury itself, is responsible for those changes. Now today, it has been demonstrated, that at least three monokines, mainly produced by cells of the monocyte-macrophage lineage, namely interleukin-1, tumor necrosis factor-alpha and interleukin-6 can mediate the acute phase response. As they can act locally as well as at distant sites in nanomolar concentrations the monokines are considered to be new hormones. Whether other newly recognized macrophage products are also involved in acute phase response has to be clarified.